The invention relates to fluid-activated primary batteries of the reserve type and to a separator system which maintains proper spacing between the positive and negative electrodes and electrically insulates the electrodes from each other.
There are a large number of nonaqueous room temperature or near room temperature battery systems that have been extensively investigated in recent years. Many of these batteries use an alkali metal such as lithium or an alkaline earth metal such as magnesium as the reducing agent at the negative electrode. A few of the systems have been produced on a limited basis for cardiac pacemakers, watches, small electronic devices and special military use. Some are designed for high energy density, high power, short life applications such as power supply for a guidance system in a military weapon. Others are designed for long life and low current drain rates such as are required in a cardiac pacemaker.
This type of battery is often built as a reserve cell for which the electrolyte system is held in reserve by packaging it separately from the cell stack and introducing it into the stack prior to use. Such cells typically comprise a sheet of alkali metal or alkaline earth metal pressed into a nickel screen or grid, a cathode conductor element typically comprising a paste of carbon powder mixed with a tetrafluoroethylene binder spread on a nickel grid, with the anode and cathode elements being separated by an inert porous material such as glass cloth filter paper, which must be permeable to permit passage of electrolyte and soluble products of decomposition of the anode.
Particularly in those applications which require both a high energy density and a high discharge rate, the cell systems must be as thin as possible with an electrode separator system which will provide a high void volume while providing sufficient integrity to eliminate electrical shorting potential between the electrodes. In addition, there is a requirement for rapid battery activation which means that the electrolyte must flow rapidly through the separator under a combination of vacuum in the dry battery stack and external pressure.
The shorting mechanism of concern is the migration, induced by mechanical shock and vibration and static electricity, of the electrically conductive carbon from the cathode conductor element to the anode through the porous separator medium. Thus, key requirements for the glass cloth filter paper separator medium include the need to be very thin, a capacity to sustain abusive handling of the battery without tearing during dry storage and a capacity to enable the battery to become activated upon injection of the electrolyte solution within a time span of less than 5 seconds, and preferably less than 1 second, without producing undesirable by-products that would interfere with the intended electrochemical reactions. While glass cloth separators would appear to satisfy these requirements, undesirable failure rates are in fact encountered in cells using such separators.
An alternative type of separator material comprises a porous, polymeric film, typically on some type of support matrix, which at least permits the passage of ions. While otherwise acceptable as a separator medium, such porous insoluble films are not, according to the patent literature, effective in preventing contact or migration of very fine carbon particles which break or slough from the cathode conductor element and bridge across the void space, through the porosities in the film. Such ionically porous films also present a quality control problem in that if too thick, they inhibit electrolyte effectiveness, and if too thin, may break and allow bridging.